This invention relates to a method of controlling the output voltage (AVR) of a pulse width modulation (PWM) inverter. More particularly, the present invention relates to a method of controlling the output voltage of a PWM inverter which is suitable for accurately controlling an induction motor (IM).
The PWM inverter is driven by a PWM signal (rectangular wave) obtained by comparing the level of a carrier wave with that of a modulated wave and outputs a voltage with the same pulse train as that of the PWM signal. Accordingly, the output voltage is also a rectangular wave and contains not only the fundamental wave component but also a large number of higher harmonic components.
When driving the induction motor at various frequencies using the PWM inverter, control is effected while the ratio of the output voltage to the frequency (E/f) is kept constant. This control system can be divided into a speed control section and a voltage control section. A detailed explanation of the speed control section is not provided herein because it is not relevant to the present invention. The conventional voltage control section has a construction in which the voltage output from the PWM inverter is detected via a power transformer (PT) and the detected value is compared with a set voltage value so as to correct the PWM signal by the difference. In accordance with this method, the detected value itself contains higher harmonic components. However, what determines the torque characteristics of an induction motor is magnetic flux and what generates the magnetic flux is the fundamental component of the output voltage. In this regard, the higher harmonic components are rather detrimental to required control. Accordingly, precision induction motor control cannot be expected if the output voltage of the PWM inverter is controlled by using the detected value, which contains higher harmonic components, as the feedback quantity.
It is therefore conceivable to isolate the fundamental component of the detected output voltage and to use it as the feedback quantity. However, this method is not practical, though theoretically possible, for the following reasons. First of all, the calculations involved in isolation of the fundamental component are so complicated that sufficient speed performance cannot be obtained, even if a high-speed Fourier converter is used. Secondly, torque variations are likely to occur if the fundamental component is isolated by a variable filter or the like. The variations become especially obvious when the inverter frequency changes rapidly. Thirdly, the voltage detecting transformer becomes saturated in the low frequency range, and therefore, output voltage detection becomes impossible in practice using known techniques.